Extraordinary efforts are often required to protect metallic articles and metal-containing articles from corrosive environments or from deterioration over long periods of time in relatively mild atmospheres. Examples of metallic articles include structures such as bridges, objects of art, and high technology devices.
Numerous kinds of corrosion inhibitors have been developed, including organic and inorganic coatings and films as well as additives to the corrosive medium. Organic inhibitors are very often derived from heterocyclic compounds. For example, benzimidazoles in corrosive solutions inhibit corrosion of metals and alloys immersed in these solutions. Metals that are protected include iron, steel, copper, brass, nickel and zinc. As an example, U.S. Pat. No. 3,222,285 teaches that solutions containing laundry detergent and an additive that is either a bisbenzimidazolyl alkane or a bisbenzimidazolyl polyhydroxyalkane are non-corrosive to brass, even though the same laundry detergent alone in solution is corrosive. Of course, addition of a benzimidazole to the environment is usually not practical, as when the corrosive environment is the atmosphere or sea water.
According to a published report by Cotton and Scholes, Brit. Corros. J., 2, pp. 1-5 (1967), monomeric benzimidazole is not effective as a corrosion inhibitor for copper in the form of a thin film because it is easily washed off of the copper by common organic solvents. Therefore, benzotriazole, films of which are not soluble, is commonly used as a film instead of benzimidazole for the prevention of corrosion of copper and brass.
Polymers can also be used as barrier coatings to protect metals from corrosion. The use of polymers with corrosion inhibiting functional groups for prevention of corrosion has also been described in the literature. For example, polyvinylimidazole, wherein the pendant imidazole moiety is an effective corrosion inhibitor, has been reported as useful for protection of copper at elevated temperatures (400.degree. C.) by Eng and Ishida in J. Mater. Science, 21, pp. 1561-1568 (1986).
Polybenzimidazoles are well-known in the art for their resistance to high temperatures, oxidation, solvents, water, and conditions that induce corrosion of metals. Several patents relating to polybenzimidazoles (U.S. Pat. Nos. Re. 26,065; 3,509,108; 3,642,720 and 3,708,439) have indicated that one of the possible applications of polybenzimidazoles would be as protective films under corrosive conditions. One polybenzimidazole variant, poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole, is commercially available from Hoechst Celanese Corporation and would therefore be exceptionally convenient. Polybenzimidazoles should make excellent protective coatings because of their stability coupled with the presence of protective benzimidazole groups, if an adherent coating can be applied to the metallic object. It is, however, well known in the art that polymer coatings on metals often do not adhere well and very often peel, which of course would leave the substrate unprotected.